Biodegradability of Drilling Fluids Used for Deep Subsurface Core Recovery in Southern McMurdo Sound, Antarctica

A deep subsurface rock core for paleoclimate reconstruction was collected in October and November 2008 during the South McMurdo Sound Project (SMS) of the Antarctic Geological Drilling Program (ANDRILL). To allow for deeper penetration and more efficient core recovery, water-based saline drilling fluids were utilized. The fluid is comprised of surface seawater from the sound, with as much as 6.5x10⁷ bacteria per liter, as the wetting agent mixed with densifying compounds (mainly potassium chloride and small amounts of thirteen other compounds including organics). In the long-term isolation from the surface geosphere and biosphere, microorganisms found in sedimentary rocks at depth are potential remnants of the ancient microbial community present while the sediments were originally being deposited. It is therefore important to understand the impact of fluids pumped into the subsurface introducing a larger and more diverse microbial community along with a readily biodegradable source of nutrients not previously available in the oligotrophic environment. In this study, drilling fluids (10.5 L) used during coring between 68 and 320 meters below the surface were collected from the drill site. These fluids were tested at in situ temperatures for microbial and geochemical changes associated with biodegradation over an eight week period. Microbial community characterization of the fluids was measured by phospholipid fatty acid analyses (PLFA) and sole carbon source utilization in BIOLOG EcoPlates (BIOLOG, Inc.) under aerobic and anaerobic conditions. Geochemical parameters measured include pH, oxidation/reduction potential, dissolved oxygen, and electrical conductivity. Fluids were also tested on a Dionex, to determine changes in the cation concentrations, and a high-performance liquid chromatograph (HPLC) to determine the change in the organic compound composition. These tests determined that the drilling fluids may impact the isolated microbial community and potentially change the geochemistry of the subsurface.